A gear transmission is known that is disposed between a motor and a work (driven member), and transmits torque of the motor to the work. Since the gear transmission deteriorates in accordance with usage, it is necessary to perform maintenance, replacement, or the like. Therefore, it is necessary to determine whether an abnormality is occurring in the gear transmission, or predict whether an abnormality will occur in the gear transmission in the near future. In Japanese Patent Application Publication No. S63-123105, a gear transmission in which an abnormality is not occurring is driven in advance under predetermined conditions, a drive pattern of the work is measured, and a reference value of the drive pattern is stored. Further, when an abnormality in the gear transmission occurs, the drive pattern of the work is measured, and a threshold value is set based on the change in the drive pattern. In Japanese Patent Application Publication No. S63-123105, after the gear transmission has operated for a predetermined time, the gear transmission is driven under the same conditions as when measuring the reference value, changes in the drive pattern of the work relative to the reference value and the threshold value are compared, and the state of the gear transmission is determined. Below, Japanese Patent Application Publication No. S63-123105 is referred to as Patent Document 1.